Lithium ion batteries provide a substantial advantage over current energy storage technology and are being developed for use in, for example, communication satellite systems, portable electronics, and electric vehicles. The use of plastic lithium ion batters (PLIB), however, can eliminate up to 500 pounds per satellite resulting in a significant cost savings and longer satellite life. The fabrication process for PLIB is a complex and lengthy procedure involving numerous stages and steps in the production cycle, each consisting of multiple, critical, and often complicated procedures.
The fabrication process for a PLIB requires the removal of plasticizers from the separator, and also from the anode and cathode, of each battery cell prior to impregnation with an electrolyte. Plasticizers may be extracted using traditional condensed solvents such as hexane, 1, 1, 1 trichloroethane, methanol, or diethyl ether, followed by several critical steps, including a rigorous drying step, to remove and avoid the uptake of water vapor or other contaminants in the extracted cell. Often, the process is individually performed on each thin layer unit cell or bicell because of the need to ensure that each cell is properly carried through the critical steps mentioned. Extraction may be performed simultaneously on a larger group of thin layer cells or bicells, but care must be taken to ensure adequate flow around each cell or bicell to ensue the required high efficiency of plasticizer and contaminant removal. The series of steps in known PLIB processes contributes to a labor intensive manufacturing process which, depending on the performance requirements of the battery, may require 20-100 or more individual unit cells or bicells stacked together for a higher energy battery cell. Also, the extraction of stacks of multiple unit cells or bicells using traditional solvents is difficult due to limited access to and migration of the extraction solvents from the cell interior. Thicker organic solvents because of their higher viscosity and lower diffusivity, have poorer mass transport properties through the porous cell structure than available with a supercritical fluid (SCF).
The use of organic extraction solvents is also under scrutiny due to problems with air pollution and ozone depletion. Finally, processes incorporating traditional solvents are not readily upgraded to multiple cell extractions and require extended extraction periods.
Following plasticizer removal and drying, contact of the extracted preform with moisture must be avoided because at this point in the manufacturing process, the anode, cathode, and separator materials are particularly vulnerable to water absorption, which adversely affects subsequent activation and final cell performance. Hours of vacuum drying at elevated temperatures is necessary to adequately remove the residual water. Therefore, the extracted cell is thoroughly dried under high vacuum and then transferred to a "dry box" where the electrolyte is introduced. The drying step is difficult and time consuming and contributes to a more complex manufacturing process, especially when processing stacks of multiple unit cells or bicells.
After drying, unit cells or bicells are currently impregnated with an electrolyte by adding a measured aliquot of electrolyte to each cell, manually massaging the cell to promote uniform absorption, followed by drying the cell with absorbent paper to remove excess electrolyte. This procedure must be performed in a "dry box," and is awkward, cumbersome and difficult to accomplish.
There are several known methods of lithium ion battery fabrication, and extraction and cleaning methods. For example, U.S. Pat. No. 5,540,741 to Gozdz et al. discloses a method making a unit cell or bicell battery. The method includes anode, cathode, and separator forming steps, as well as an extraction step to remove the bulk of the plasticizers from the anode, cathode, and separator layers of the unit cell. The plasticizer/contaminant extraction step preferably utilizes a liquid solvent. An alternative extraction method for single cells is disclosed using supercritical fluids such as carbon dioxide, propane, or nitrous oxide at temperatures above their respective critical points. At supercritical temperatures, the liquid and gas phases cease to separately coexist, resulting in the formation of a single phase. Additionally, the use of entrainers or cosolvents is disclosed, particularly with the use of carbon dioxide in supercritical fluid extraction (SFE). Although SFE is generally known as a potential method, Gozdz does not specifically teach a viable method of SFE in conjunction with battery fabrication.
Also, U.S. Pat. No. 5,013,366 to Jackson et al. discloses a cleaning process for removing two or more contaminants using temperature variable phase shifting of dense gas phases.
Accordingly, there is a present need to simplify manufacturing of PLIBs to eliminate the environmental risks and liabilities attendant in the use of traditional extraction solvents, and to shorten the extraction periods and also provide for multiple cell extraction.